(a) Field of the Invention
This invention relates to a rubber-modified styrene resin having excellent transparency and good impact resistance and to a production process thereof. More specifically, this invention is concerned with a transparent, rubber-modified styrene resin obtained by polymerizing a styrene monomer and an alkyl (meth)acrylate to a high degree of polymerized conversion in the presence of a rubbery polymer having a specific structure in accordance with an extremely simple polymerization process so that the rubbery polymer is formed into particles while allowing a styrene copolymer having substantially the same refractive index as the rubbery polymer to occur, and with the production process for the transparent, rubber-modified styrene resin.
(b) Description of the Related Art
Styrene resins are used for a wide variety of applications, led by home electric and electronic appliances, office automation equipments and packaging materials, for their characteristic properties of excellent transparency, stiffness and moldability. Use of a styrene resin alone however cannot provide sufficient impact resistance so that, for applications where impact resistance is required, the styrene resin is generally used after modifying it with a rubbery polymer. This rubber-modified styrene resin has been significantly improved in impact resistance compare with the unmodified resin but is no longer equipped with the inherent characteristic property of the styrene resin, that is, transparency. This is attributed to the difference in refractive index between the styrene resin and the rubbery polymer.
According to J. Brandrup and E. H. Immergut, Polymer Handbook, 3rd ed., VI, 451-461, John Wiley & Sons, New York, N.Y. (1989), the refractive index of polystyrene is 1.59 to 1.592 while that of polybutadiene is 1.516 to 1.520. In general, a styrene resin has a higher refractive index than a rubbery polymer.
However, there is still a strong market demand for the transparency of styrene resins. It is therefore an extremely important theme from the industrial standpoint to develop a transparent, rubber-modified styrene resin, in other words, to retain the transparency of a styrene resin while imparting impact resistance to it.
Upon using a transparent, rubber-modified styrene resin especially in the field of packaging materials, the resin is first formed into a sheet or the like, followed by secondary processing into a desired shape in accordance with vacuum forming or pressure forming. This secondary processing however causes the transparency of the resin to be lowered. There is accordingly a great market demand not only for the transparency of the resin itself but also for the retention of transparency upon its forming or shaping.
Reflecting ever-increasing concern for environmental problems in recent years, there is an increasing desire toward resins which permit easy material recycling. Development of a resin permitting easy material recycling is also important from the viewpoint of productivity because, when a resin sheet is subjected to secondary processing to form, for example, blister cases or the like, higher productivity can be achieved by using the resin, which still remains after punching out the processed products, again as a molding or otherwise forming material. To recycle a resin as a natural resource, the once-processed resin is ground, is heated, molten and kneaded by an extruder or the like, and is then used again as a material. A conventional transparent, rubber-modified styrene is accompanied by the problems that when repeatedly heated, the color of the resin changes and the impact resistance becomes lower. For the development of a transparent, rubber-modified styrene resin, it is accordingly an important theme now to prevent deterioration of its physical properties, especially, a change in its hue and a reduction in its impact resistance upon heating.
Conventional transparent, rubber-modified styrene resins are produced, for example, by blending a polystyrene as a styrene resin with a styrene-butadiene block copolymer as a rubbery polymer as disclosed in Japanese Patent Laid-Open No. 195139/1982. However, products obtained by such blending however cannot successfully meet the market demand in view of the problems to be described next. Namely, if the blending method or condition for the polystyrene and the styrene-butadiene block copolymer is changed (for example, the type of an extruder in which the blending is to be performed is changed from the single screw type to the twin screw type even when the same materials are employed, or the extrusion temperature, residence time, rotational speed and the like are changed although the same materials and the same extruder are used), the kneaded state, the thermal deterioration and/or the degree of discoloration of the two materials change, resulting in changes in both transparency and impact resistance. This leads to a problem in reproducibility. Depending on the extrusion conditions, the rubbery polymer may be gelled, leading to serious problems from the standpoint of molding and/or processing such as occurrence of fish eyes. Moreover, their refractive indices are basically different so that there is naturally a limit to the transparency of the resulting product.
From the viewpoint of material recycling, the deterioration of the resin significantly proceeds to lower the impact resistance as the heat history becomes longer. This is certainly not preferred. To improve the transparency, it has also been proposed to make the content of styrene higher in the styrene-butadiene block copolymer to be blended. The copolymer however is imparted with properties similar to polystyrene so that physical properties, especially, impact resistance of the product is extremely lowered. This approach is therefore not preferred.
With a view to overcoming the above-described problems, Japanese Patent Laid-Open No. 351649/1992, for example, discloses a process in which a styrene copolymer, which has been obtained by copolymerizing one or more alkyl acrylates or alkyl methacrylates with a styrene monomer and has a refractive index similar to a rubbery polymer, is blended with the rubbery polymer.
The important point of the production of the copolymer having a similar refractive index as the rubbery polymer resides in controlling the ratio of styrene monomer structural units to alkyl (meth)acrylate structural units, said former and latter structural units making up the styrene copolymer, in conformity with the molecular structure of the rubbery polymer so employed.
Assuming, for example, that the rubbery polymer is a styrene-butadiene copolymer, the styrene-butadiene copolymer is required to satisfy the following formulae (1), (2) and (3): EQU .vertline.n.sub.1 -n.sub.2 .vertline..ltoreq.0.01 (1)
where EQU n.sub.1 =0.01591.times.X+0.01518.times.(100-X) (2) EQU n.sub.2 =n.sub.s .times.Y.div.100+n.sub.M .times.(100-Y).div.100(3),
X: the mole % of styrene structural units contained in the rubbery polymer, PA1 Y: the mole % of styrene monomer structural units contained in the styrene copolymer, PA1 n.sub.s : the refractive index of the structural unit of the styrene monomer, and PA1 n.sub.M : the refractive index of alkyl (meth)acrylate structural units. PA1 (1) 70 to 96 parts by weight of a copolymer (A) formed of 20 to 70 wt. % of styrene monomer units and 30 to 80 wt. % of alkyl (meth)acrylate monomer units, and PA1 (2) 4 to 30 parts by weight of a rubbery polymer (B), said rubbery polymer being dispersed in said copolymer (A) as particles having an average particle size of 0.1 to 2.0 .mu.m, at least 70 wt. % of said rubbery polymer being a styrene-butadiene block copolymer (B1) which is formed of 5 to 50 wt. % of styrene units and 50 to 95 wt. % of butadiene units, has a viscosity in a range of 3 to 60 cps when measured as a 5 wt. % styrene solution at 25.degree. C. and possesses a ratio (Mw/Mn) of a weight average molecular weight (Mw) to a number average molecular weight (Mn) in a range of 1.0 to 1.8, PA1 wherein said copolymer (A) and said rubbery polymer (B) have substantially the same refractive index. PA1 continuously charging into a polymerizer polymerization raw materials composed primarily of (B) a rubbery polymer, a-styrene monomer and an alkyl (meth)acrylate, said rubbery polymer (B) comprising at least 70 wt. % of (B-1) a rubbery polymer which is a styrene-butadiene block copolymer formed of 5 to 50 wt. % of styrene units and 50 to 95 wt. % of butadiene units, has a viscosity of 3 to 60 cps when measured as a 5 wt. % styrene solution at 25.degree. C. and possesses a ratio (Mw/Mn) of a weight average molecular weight (MW) to a number average molecular weight (Mn) in a range of 1.0 to 1.8; PA1 polymerizing the raw materials at a polymerization temperature of 80.degree. to 160.degree. C. under continuous stirring of the resulting polymerization mixture to form (A) a copolymer of the styrene monomer and the alkyl (meth)acrylate having a refractive index substantially the same as the rubbery polymer (B), and under the condition that the solid content of the polymerization mixture is not higher than 60 wt. %, causing the copolymer (A) and the rubbery polymer (B) to transform into a continuous phase and a dispersed phase, respectively, so that the rubbery polymer (B) is formed into dispersed particles having an average particle size of 0.2 to 2.0 .mu.m; and PA1 feeding the polymerization mixture into a volatile elimination unit to eliminate any unreacted monomers from the polymerization mixture.
The values of n.sub.s and n.sub.M in the formula (3) can be found in a publication such as J. Brandrup and E. H. Immergut, Polymer Handbook 3rd ed. , VI, 451-461, John Wiley & Sons, New York, N.Y. (1989) on the basis of the types of the monomers employed.
It is therefore possible to determine the ratio of the styrene monomer structural units to the alkyl (meth)acrylate structural units in the styrene copolymer in accordance with the formulae (1), (2) and (3).
The above process can improve the transparency to some extent because the styrene copolymer and the rubbery polymer have substantially the same refractive index. With respect to the impact resistance, especially, the impact resistance after repeated heating, the problem has not been improved at all because the process relies upon blending. Namely, the resulting blend has low impact resistance and moreover, its impact resistance considerably varies depending on the degree of kneading and is progressively lowered as it is repeatedly heated. The blend is therefore not preferred.
A process has hence been proposed to produce a transparent, rubber-modified styrene resin by subjecting a styrene monomer and an alkyl (meth)acrylate at a particular ratio to emulsion polymerization in the presence of a rubbery polymer latex as disclosed, for example, in Japanese Patent Publication No. 43450/1987.
Further, as disclosed in Japanese Patent Laid-Open No. 224848/1992, it has also been proposed to produce a transparent, rubber-modified styrene resin by blending in an extruder or the like a graft polymer--which has been obtained by polymerizing a styrene monomer and an alkyl (meth)acrylate with a rubbery polymer obtained by emulsion polymerization--with a copolymer of a styrene monomer and an alkyl (methacrylate) produced by bulk or solution polymerization. According to these processes, the impact resistance has been far improved over the blending process, because the rubbery polymer exists as stable particles adequately crosslinked in advance with divinylbenzene or the like and a portion of the copolymer of the styrene monomer and the alkyl (meth)acrylate formed by the polymerization has been grafted on the rubbery polymer. Because of the production process, however, an amphiphatic low-molecular compound called an "emulsifier" has to be added to emulsify and disperse the hydrophobic monomers in water upon polymerization and moreover, a coagulant such as calcium chloride has to be added subsequent to the completion of the polymerization in order to separate the resultant polymer latex from water. The resin so produced therefore contains impurities such as the emulsifier and the coagulant so that the resin has extremely low transparency due to these impurities. Further, upon molding such resins, impurities remain on a mold. The impurities deposited on the mold are discolored as molding is repeated. Eventually, the impurities are transferred onto the resin, resulting in molded products having poor external appearance. In addition, when a vent portion of the mold is clogged by such impurities, gas is not released from the mold. Therefore the mold is no longer uniformly filled with the resin so that a short shot takes place. To avoid such poor external appearance or short shot, the mold is usually washed periodically. Deposit of such impurities in a large quantity hence requires frequent washing, resulting in a reduction in productivity. To reduce such impurities, it may be contemplated to repeatedly and thoroughly wash the resin in the coagulation step of the resin production process. This measure is however not preferred for a higher production cost, because it requires additional production equipments, a great deal of water for washing and tremendous labor for the treatment of waste water.
To eliminate impurities such as an emulsifier contained in a resin, it has been proposed to change polymerization from emulsion polymerization to bulk or solution polymerization.
Japanese Patent Publication No. 25215/1980 discloses to produce a transparent, rubber-modified styrene resin by dissolving a rubbery polymer in a monomer composed of methyl methacrylate and styrene, and then subjecting the resulting solution to batchwise bulk polymerization. The rubbery polymer is a substance which has ability to react with radicals and is rubbery at room temperature. Usable examples of the rubbery polymer include polybutadiene, butadiene-styrene random copolymers, and butadiene-styrene block copolymers. This process does not use an emulsifier or the like which poses a problem in the production of a resin by emulsion polymerization or bulk-suspension polymerization, so that the above-described impurity-related problem does not arise. According to the specification as published, however, the resulting rubbery polymer has a particle size as large as 4 to 7 .mu.m so that the molded or otherwise formed product has rough surfaces. Scattering of light therefore takes place, resulting in low transparency. In addition, the transparency is also lowered by molding or processing. The resin is also accompanied by the drawbacks that compared with the impact resistance and hue of the resin before heating, its impact resistance and hue after heating are significantly lower and substantially different.
To improve a reduction in transparency due to a distribution in the copolymerized composition, Japanese Patent Publication No. 31488/1988 (corresponding to U.S. Pat. No. 4,230,833) discloses production of a transparent, rubber-modified styrene resin by continuous-flow bulk polymerization. Described specifically, a polymerization raw material formed by dissolving a rubbery polymer in a monomer composed principally of methyl methacrylate and styrene is continuously supplied to a single reaction tank. While continuously stirring the solution and controlling the temperature, pressure and average residence time at 161.degree. C. to 195.degree. C., 100 to 175 psig and less than 90 minutes, respectively, polymerization is conducted to produce a transparent, rubber-modified styrene resin. Usable examples of the rubbery polymer include polybutadiene, butadiene-styrene copolymers, butadiene-acrylonitrile copolymers, ethylene-propylene-diene copolymers, and isoprene polymer and copolymers.
Since this process makes use of continuous-flow bulk polymerization and the polymerization is conducted in the single reaction tank, the copolymer so formed has a narrow composition distribution. This process is therefore considered to be suited for providing a resin with transparency. Production of a resin on the basis of this process however requires extremely precise control of polymerization conditions such as polymerization temperature, reaction pressure and average residence time. Further, the polymerization reaction tends to run away as the polymerization temperature is as high as 161.degree. C. to 195.degree. C. It is therefore difficult to maintain the polymerization conditions within the prescribed ranges for a long period of time. To practice this process as an actual plant, there is accordingly a serious problem in the operation stability of the plant, hence, in the ability to stably produce a resin having high transparency and impact resistance. Moreover, a great deal of labor is needed for the control of the polymerization conditions so that the production cost is high. In addition, the high polymerization temperature results in the formation of low m.w. copolymers in large quantities. Like impurities contained in a resin produced by the above-described emulsion or suspension polymerization, these low m.w. copolymers cause a reduction in the transparency of the resin and also induce discoloration or a short shot upon molding or processing.
As a production process of a transparent, rubber-modified styrene resin by another continuous-flow bulk polymerization, Japanese Patent Laid-Open No. 180907/1992, for example, discloses to use a styrene-butadiene copolymerized rubber containing 1,2-vinyl bonds in a proportion of 14 to 25% based on unsaturated bonds derived from butadiene and to conduct polymerization under static mixing of a polymerization mixture in an apparatus with a built-in tubular reactor in which plural mixing elements having no movable parts are internally fixed. This process features easy controllability of the size of rubber particles. However, the tubular reactor which makes up a recirculation line and a non-recirculation line for the polymerization mixture has lower mixing ability for the polymerization mixture compared with a complete-mixing reaction tank. The composition of the polymerization mixture therefore becomes uneven, resulting in the formation of a copolymer of a styrene monomer and a (meth)acrylate ester with a broader composition distribution. On the other hand, use of plural reactors requires a great deal of labor for the control of the copolymer composition because the mixing ratio of the unreacted styrene monomer to the unreacted (meth)acrylate ester varies from one reactor to another. Accordingly, the resulting styrene resin is not significantly improved in transparency compared with that produced by a batchwise reaction. Further, any attempt to increase the concentration of the rubbery polymer in the resin requires to substantially lower the degree of polymerized conversion in each reactor depending on the number of reactors, thereby posing problems on the controllability of the polymerization and the productivity at that time.
As a production process of a transparent, rubber-modified styrene resin by further continuous-flow bulk polymerization, Japanese Patent Publication No. 54484/1993, for example, discloses a process for the production of a transparent, rubber-modified styrene resin, which features mixing a first flow--which has been obtained by polymerizing a solution of a rubbery polymer, a styrene monomer, an alkyl (meth)acrylate and a solvent and terminating the polymerization at such a degree not exceeding a degree of polymerized conversion beyond which the rubbery polymer is converted into particles--with a second flow of a solution of the styrene monomer, the alkyl (meth)acrylate and the solvent, the latter solution being in the course of polymerization, and polymerizing the thus-obtained mixture to form the rubbery polymer into particles. As the rubbery polymer, one or more rubbery polymers selected from polybutadiene and butadiene-styrene copolymers can be used. According to this process, polymerization is conducted to an appropriate degree in each of plural reactors. Since the mixing ratio of the unreacted styrene monomer to the unreacted alkyl (meth)acrylate varies from one reactor to another, the resulting copolymer of the styrene monomer and the alkyl (meth)acrylate however tends to have a broad composition distribution. There is accordingly a problem in that strict control of polymerization reactions is needed for the assurance of transparency. Further, nothing is taken into consideration for the maintenance of hue and impact resistance after heated.
Japanese Patent Laid-Open No. 145443/1994 discloses a styrene resin composition which has been obtained by blending a rubber-modified styrene polymer with a styrene-butadiene block copolymer and a terpene resin and which undergoes less reduction in transparency upon molding or processing, especially upon secondary processing. As the styrene-butadiene block copolymer and the terpene resin are added in this process, the production cost of the resin composition is high and no improvement has been made for the variation in hue after heated. In addition, the impact resistance progressively lowers as the resin composition is repeatedly heating. This process is therefore not preferred.
To obtain a transparent, rubber-modified styrene resin having high impact resistance, the conventional art is recognized, as described above, to have changed the production process from emulsion polymerization or bulk-suspension polymerization to batchwise bulk or solution polymerization or continuous-flow bulk or solution polymerization and also to have investigated additives. However, neither a rubber-modified styrene resin composition achieved both possession of transparency and impact resistance and prevention of a reduction in transparency upon molding or processing and a change in hue and a reduction in impact resistance even when heated nor a production process therefor has been developed yet.